This invention relates to sirens. In particular, the invention is directed to a siren for circumferentially radial distribution of acoustic output for alerting communities, for instance, in emergencies at nuclear power stations or in events of other calamity.
Sirens can be of an integral blower-type siren where the sound generation includes an internal air compressor-rotary valve combination, and this inherently of low efficiency. The alternative siren design employs an axial flow which includes an external compressor. Although it incorporates efficient compression, it is only unidirectional, and the bending of sound into the radial horizontal plane creates inefficiencies such that the horizontal plane acoustic power generation is reduced. Turbulence of air in such a siren acts as a pneumatic or acoustic resistance to the siren.
As such therefore known sirens for warning and alerting operate at a relatively low acoustical efficiency. The efficiency is a measure of the acoustical output, usually in the horizontal plane, relative to the electrical or mechanical input power. In the Applicant's experience, this efficiency varies between 3% to 10% for commercially available sirens. In the known sirens where the acoustical output is generated in an axial direction, usually upwardly, a horn or the like is provided for turning the acoustical output to radiate in a horizontal direction. For a siren to be heard over a wider geographical area, it is desirable to radiate the acoustical output horizontally, and many of the commercially available sirens do not provide an internal mechanism for inherently creating such horizontal radiation. It is only by the provision of the appendaged horn that this horizontal radiation is achieved.
The redirecting of this radiation pattern, either through the use of deflectors or bent tubes or horns, or by some similar guiding, reflecting or defracting mechanism, all result in a loss of available sound energy in the horizontal plane, compared to an inherently radially-radiating siren. This redirection of acoustical output impairs the acoustic efficiency of the siren performance.
A further problem which is encountered in known sirens is that the mechanism within the sirens generating the sound is of a nature which causes excessive turbulence of the compressed gas or air passing through the siren mechanism, such that the acoustical output and the efficiency is further reduced.
Furthermore, known sirens do not provide an efficient or adequate degree of a sealing action between moving parts such that leakage of compressed air between moving parts further impairs the output efficiency and causes turbulence within the acoustical generating mechanism.
When the relatively rotational ports are not in alignment, namely, the ports are closed, ideally no air should flow outwardly to the siren horn. In actual fact, there is always some air flow or leakage, and this leakage is a significant source of lost siren efficiency. The space between the inside wall of a stator member and outside wall of a rotor member is often only a few thousandths of an inch or less, but even with such close spacing the loss of efficiency is significant. Where in commercial, community-type warning sirens such close clearances are impractical, the losses are even higher. The seal for an application to a siren where there is relatively high speed between the inside face and the ouside wall of the stator and rotor, respectively, such speed being in the order of 10,000 feet per minute, or greater, presents a difficulty since this generates unacceptable heat and/or friction where the seal comes into contact with the stationary face of the stator. This heat and/or friction tends to destroy the seal and/or the rotor or stator, or to increase the torque requirements to unacceptable levels.
Another problem with sirens arises in the desirability to radiate the sound uniformly in the horizontal plane. This is often accomplished by employing four or more horns to distribute the sound as uniformly as possible in the circumferential horizontal plane.
Where there are spaced ports for outlets for horns circumferentially around the location of the siren the acoustical output generated from the one horn effectively diminishes or cancels the acoustical output from adjacent horns so that at locations remote from the siren the acoustical output is consequently diminished and the efficiency of the reception is reduced.
At any given observation point, the sound field will originate not only from the horn pointing most directly towards the observer, but also from all the other horns. Since the effective sources of sound are near the mouths of the horns, the sound from each horn will travel a distance dependent upon the relationship between the observation point and the horn geometry. With the observation point directly in line with one horn, there will be a series of siren-to-observer distances at which the sound from the two horns adjacent to the centrally positioned horn will travel exactly one-half of the acoustic wavelength, for the particular siren frequency, farther than the sound from the central horn.
The sound from the central horn would be exactly 180 degrees out of phase with the sound from the adjacent horns. Thus, if the siren has only three horns and the level from each off-axis horn was 3 dB less than that from the central horn at the observation point, complete cancellation would result and the sound level would be zero. At some other observation distance, the path length difference would be 1 wavelength, and the sound level would be 3 dB greater than if only one horn were radiating. Thus, the level would fluctuate from zero to 3 dB more than that from one horn alone. Similarly, if the observer traveled in a circle about the siren, the level would fluctuate as the relationship between the path length changed due to the changing geometry. A similar or somewhat more complex effect occurs when the siren has more than three horns. At a constant measurement distance from the siren, the level may fluctuate several dB above and below the median value.
The result is that the alerting effectiveness is less at some locations that at others the same distance from the sirens. With the horn arrangement of the invention, these undesirable acoustic characteristics are reduced, not by rotating the horns, which would result in undesirable mechanical reliability problems, but through internal design.
Accordingly, the distance from which a siren may effectively be heard will be markedly affected and reduced by these inefficient operating characteristics in known sirens.
There is accordingly a need to provide a siren which minimizes the above problems and provides a more efficient acoustical output, and, for this purpose, to minimize the air turbulence generation within the siren, and to insure that leakage of compressed air between moving parts is minimized. Furthermore, it is desirable to provide a siren where the acoustical outputs generated by different output ports of the stator are in a phase relationship relative to each other so that they complement each other, that in a spatial distribution at a removed distance from the siren the effective sound generation is additive and hence more efficient and more uniform.